Ephesians 3:1-9: “For this cause I Paul, the prisoner of Jesus Christ for you Gentiles, If ye have heard of the dispensation of the grace of God which is given me to youward: How that by revelation he made known unto me the mystery; (as I wrote afore in few words, Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit; That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel: Whereof I was made a minister, according to the gift of the grace of God given unto me by the effectual working of his power. Unto me, who am less than the least of all saints, is this grace given, that I should preach among the Gentiles the unsearchable riches of Christ; And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ.”
What you miss is that the Church is not a New Testament novelty introduced by Christ but an ongoing spiritual organism that has contained the elect of God from the very beginning. The Church is not something entirely unique in God's plan and purposes but is an extension of Old Testament believing Israel. Whilst the Church has taken on a different form under the new covenant, in the same way as the development / change occurs between the caterpillar and the butterfly, the elect in the Old Testament and the elect in the New Testament are part of the same spiritual body.
Paul never says that the Church wasn’t about before Pentecost. In fact he teaches the opposite. He identifies the mystery in a clear and unambiguous way in verse 6, namely: “That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel.” The Dispensational interpretation is the exact opposite to what the inspired text is actually saying. Paul is in fact talking about the joining of the old and new covenant saints together in Christ. The mystery is the mystical union of the people of God of all time in one spiritual body. He is talking about the parity that resulted from this merger in regard to the promises of God.
The Church itself was not a mystery (or secret) prior to Paul, neither was God's great eternal plan of redemption, neither was the ingathering of the Gentiles. Passage after passage in the Old Testament predicted these events. What was a mystery was the Gentiles being “fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel.”
